STAC9766 datasheet(21/97 Pages) IDT

21 / 97 page

STAC9766/9767

TWO-CHANNEL, 20-BIT, AC’97 2.3 CODECS WITH STEREO MICROPHONE AND MIC/JACK SENSING

PC AUDIO

IDT™

STAC9766/9767

V 7.4 12/06

TWO-CHANNEL, 20-BIT, AC’97 2.3 CODECS WITH STEREO MICROPHONE AND MIC/JACK SENSING

Figure 12. CODEC Clock Source Detection

The STAC9766/9767 uses the XTAL_OUT pin (pin 3) and the CID0 and CID1 pins (pins 45 & 46) to

determine its alternate clock frequencies. See section 2.2.4: page14 for additional information on

Crystal Elimination and for supported clock frequencies.

If, when the RESET# signal has been de-asserted, the CODEC has not detected a signal on

BIT_CLK, as defined in the previous paragraph, then the AC‘97 CODEC derives its clock internally

from an externally attached 24.576 MHz crystal or oscillator (or optionally from an external

14.318 MHz oscillator), and drives a buffered 12.288 MHz clock to its digital companion Controller

over AC-link under the signal name “BIT_CLK”. Clock jitter at the DACs and ADCs is a fundamental

impediment to high quality output, and the internally generated clock will provide AC‘97 with a clean

clock that is independent of the physical proximity of AC‘97’s companion Digital Controller (hence-

forth referred to as “the Controller”).

If BIT_CLK begins toggling while the RESET# signal is still asserted, the clock is being provided by

other than the primary CODEC, for instance by the controller or by a discrete clock source. In this

case, the primary CODEC must act as a consumer of the BIT_CLK signal as if it were a secondary

CODEC.

AC'97 Clock Source

Detection

RESET# Signal Asserted

BIT_CLK Toggling?

12.288MHz signal on BIT_CLK is

being generated externally; codec

uses this signal as the clock.

Yes

After RESET# Signal

Deasserted

24.576MHz

crystal present?

24.576MHz Crystal on XTL_IN

and XTL_OUT used by codec to

generate clock on BIT_CLK

Yes

24.576MHz

oscillator present?

24.576 MHz signal on XTL_IN

used by codec to generate

12.288MHz clock on BIT_CLK

Yes

14.318MHz

oscillator presnent?

14.318 MHz signal on XTL_IN

used by codec to generate

12.288MHz clock on BIT_CLK

Yes

Error condition - no clock

source present

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